1. Field of the Invention
This invention relates to an X-ray diagnostic apparatus in which an X-ray tube voltage can be stabilized.
2. Description of the Prior Art
In a conventional X-ray diagnostic apparatus, it is most important to stabilize an X-ray tube power output as an information source in order to obtain more accurate diagnostic information. An X-ray tube is generally used as an X-ray radiation source. A high voltage (i.e., tube voltage) applied across the two electrodes of the X-ray tube and heating of an X-ray tube filament must be stabilized so as to realize a stable X-ray tube power output.
In an X-ray diagnostic apparatus of an X-ray generation system wherein a maximum permissible initial emission tube current preset in accordance with an object to be examined is reduced in approximation along a load characteristic curve (referred to as a "falling load") simultaneously while a picture is being taken, the initial preset value of the tube voltage will increase from time to time. The tube voltage may often exceed a maximum rating tube voltage of the X-ray tube. In a conventional X-ray diagnostic apparatus, in order to solve the above problem, the tube voltage is decreased in a stepwise manner every time a given short time period has elapsed, so that the tube voltage is kept constant. According to this method, a decrease in tube voltage must be continuously controlled as a function of time. For this purpose, the tube voltage is lowered by electromagnetic switches and line resistors. However, an optimum response cannot be obtained. Therefore, it is difficult to obtain a stable tube voltage and, hence, a stable X-ray tube power output.
FIG. 1 is a block diagram of a conventional X-ray diagnostic apparatus employing a stepwise falling load system. When a line power switch 1 connected in a low voltage source 60 of the X-ray diagnostic apparatus is turned on, a low input voltage is applied to a slidable autotransformer 2. When the tube voltage of the X-ray tube is set by a program unit 3 for controlling X-ray emission, a conductive slidable roller 6 of the slidable autotransformer 2 is controlled such that the primary voltage is regulated through an amplifier (referred to as "AMP") 4 by a DC servo motor to correspond to the tube voltage of the X-ray tube. The maximum permissible initial emission tube current can be preset by the program unit 3 using the preset tube voltage and the load characteristics of the X-ray tube used. A timer 7 is started to execute a tube current timer control program wherein the load or the tube current corresponding to the preset tube emission current is reduced in accordance with the load characteristics. Upon operation of the timer 7, the initial tube emission current is set at the primary winding side of a filament heating transformer 11 through a relay (referred to as "RY") 8. The primary winding side is constituted by a stabilizing power source (referred to as "SPW") 12 for stably controlling heating of the filament and a filament heating resistor 13. The tube emission current is controlled by the timer 7. Since the filament heating resistor 13 is controlled to decrease the tube current at predetermined short timing periods, RYs 9 and 10 are controlled for each X-ray emission in accordance with the program of the timer 7 in the same manner as is the RY 8. The tube voltage changes during X-ray emission, or exposure in synchronism with a falling load time (i.e., a stepwise time interval during which the tube emission current is changed by the RYs 8, 9 and 10) set by the timer 7. A tube voltage changing circuit (to be referred to "VSW") 14 is thus operated by the program unit 3. At the initial period of X-ray emission, all RYs 15, 16 and 17 are closed by the VSW 14, and hence all line resistors 18, 19 and 20 are directly connected to the main circuit.
When X-ray exposure is started under the above-described conditions, an X-ray exposure control circuit 21 is actuated, a main switch 22 is closed, and then a line voltage is applied to a high-tension transformer 23. Meanwhile, the RY 8 is closed in accordance with the program of the timer 7, and the filament of the X-ray tube 25 is heated. A high AC voltage is applied from the high-tension transformer 23 to a high-tension rectifier (referred to as "RECT") 24. The rectified voltage is then applied to the X-ray tube 25. An X-ray is emitted from the X-ray tube 25 and irradiates an object to be examined (referred to as "OBJ") 26. An X-ray picture of the OBJ 26 is formed on an X-ray film 28 through an ionization chamber 27 for automatic exposure control.
During X-ray exposure, when falling load time t.sub.1 (e.g., 0.1 s) is reached in accordance with the program of the timer 7, the RY 8 is opened and at the same time the RY 9 is closed. Furthermore, the RY 15 is opened. At falling load time t.sub.2 (e.g., 1.0 s), the RY 9 is opened and at the same time the RY 10 is closed. Furthermore, the RY 16 is opened.
The tube current of the X-ray tube is decreased in accordance with the program of the timer 7 as the exposure time elapses. During X-ray exposure, the X-ray output power is detected as an X-ray exposure dosage by the ionization chamber 27. The detected exposure dosage is compared by a comparator (referred to as "COMP") 29 (FIG. 4) with a reference blacking level preset by the program unit 3. When the detected exposure dosage reaches the reference blacking level, the COMP 29 supplies an X-ray emission interrupting signal to the X-ray exposure control circuit 21. The main switch 22 is opened by the X-ray exposure control circuit 21, thereby interrupting X-ray exposure.
FIG. 2 shows a graphical representation of the emission current and the X-ray voltage as a function of exposure time in the X-ray diagnostic apparatus of a falling load system. Referring to FIG. 2, a curve "a" indicates the emission current which is decreased along the exposure time base; a curve "b" indicates the X-ray tube voltage when the tube voltage is not controlled; and a curve "c" indicates the X-ray tube voltage when ideal tube voltage control is performed. However, an actual X-ray tube voltage in the conventional X-ray diagnostic apparatus has a large ripple amplitude as indicated by a curve "d" in an enlarged representation shown in FIG. 3.
In the conventional X-ray diagnostic apparatus employing the falling load system, since the load is mechanically and intermittently decreased by insertion of the line resistor, the X-ray tube voltages respectively indicated by the curve "c" (FIG. 2) and a curve "e" (FIG. 3) cannot be obtained. In the conventional control system, since the tube voltage can be set as large as permitted by the maximum rating voltage of the X-ray tube, the X-ray tube may then be broken when the tube voltage varies during X-ray irradiation. When the tube voltage varies during X-ray irradiation, a wave length (.lambda.) of the X-rays during X-ray exposure may change (FIG. 3), and the X-ray absorbing conditions in the OBJ may change (depending on fat, soft tissue, and bone), degrading X-ray image quality. Furthermore, in the case of forming a conventional tomograph, the tube voltage irregularly changes at tomographic imaging rotational angles. This may result in degradation of the X-ray image quality.